1. Field of the Invention
The present invention relates to an insulated gate semiconductor device used for power control. More specifically, the invention relates to a MOS gate device such as a switching power MOSFET (metal oxide semiconductor field effect transistor) and an IGBT (insulated gate bipolar transistor), or a MIS (metal insulated semiconductor) gate element.
2. Description of the Related Art
In miniaturizing a power supply circuit such as a switching mode power supply, it is effect to increase switching frequency. In other words, it is effective to downsize a passive element such as an inductance and a capacitor in a power supply circuit. However, as the switching frequency is increased, a switching loss increases in switching elements such as a MOSFET and an IGBT. The increase in switching loss lowers the efficiency of a power supply. Therefore, in order to miniaturize the power supply circuit, it is essential to decrease the switching loss due to a speedup of switching elements.
In MOS gate elements, such as a MOSFET and an IGBT, currently used as switching elements, a gate length is shortened thereby to decrease the opposing area of gate and drain electrodes. Consequently, it is possible to increase the speed of the MOS gate elements by reducing gate-to-drain capacitance.
However, when the gate-to-drain capacitance is reduced to speed up the MOS gate elements, resonance occurs between parasitic inductance and switching element capacitance contained in wiring. The resonance becomes a factor in causing high-frequency noise (switching noise) at the time of switching. In order to suppress the switching noise, it is necessary to perform soft switching, or provide a filter circuit, or devise a gate drive circuit. The suppression of switching noise increases costs.
As described above, conventionally, it is possible to achieve high-speed switching by reducing gate-to-drain capacitance. However, it is necessary to suppress switching noise. For this purpose, it is necessary to perform soft switching or use an external circuit such as a filter circuit.